Dynamometer testing of vehicles is known per se, and can, for example, be carried out by roller type (rolling road) dynamometers equipped with large rollers that support the vehicle wheels, and which are used to apply a brake torque to the drive wheels of the vehicle. Such systems, however, are not always capable of providing the desired measurement accuracy and/or freedom of measurement.
Another kind of vehicle dynamometer systems for dynamometer testing of vehicles is disclosed in U.S. Pat. No. 4,669,318 (Ångstrom). This document relates to an apparatus for dynamometer testing of vehicles, where load absorbing means in the form of a hydrostatic pump assembly has an input shaft for engagement with a drive shaft of a vehicle to be tested. Each drive shaft is fixedly connected to an individual apparatus of this kind, whereby a total effective torque from the vehicle can be accurately measured.
It is also possible to perform more complex tests using a dynamometer testing system of the above kind, both for two-wheel drive systems, and also for four-wheel drive systems. Such more complex testing, using a system of the above kind, is disclosed in the International patent application WO2007/133154 A1 (Engstroem).
However, vehicle transmissions are becoming increasingly complex, and may include various kinds of power sources for providing power to wheel shafts of the vehicle. These power sources can be arranged to provide propelling powers, but also braking powers, e.g. when used for regenerative braking. The increased complexity of vehicle transmissions provides corresponding challenges for dynamometer testing systems. Furthermore, there is also an increasing desire to perform measurements that allows testing taking into account situations that may occur during real life driving of the vehicle on a road, such as climate related conditions that may affect the behavior of the vehicle.
Aim and Most Important Features of the Invention
It is an object of this invention to provide a method for use in dynamometer testing of vehicles that allows use of cost-efficient dynamometer solutions when testing vehicle behavior in various conditions.
According to the present invention, it is provided a method for use in dynamometer testing of a vehicle, the vehicle including at least a first wheel shaft and at least one first vehicle power source for providing power to said first wheel shaft, said first wheel shaft being connected to a vehicle dynamometer system, said vehicle dynamometer system comprising a first controllable dynamometer power source for providing power to said first wheel shaft, said first dynamometer power source being an electrical machine comprising a stator and a rotor, said stator comprising a stator winding and said method including:                determining whether a first temperature is below a first temperature limit, and        heating said electrical machine by applying a current to said stator winding when said first temperature is below said first temperature limit.        
As was mentioned above, there is an increasing desire to perform dynamometer measurements of vehicles that allows accurate measurement of various vehicle characteristics taking into account numerous situations that may occur during actual driving of the vehicle on a road. Vehicle testing in general often includes testing in various conditions, for example, in extreme weather conditions, e.g. very warm or very cold conditions. These tests are often performed in locations exhibiting the desired climate, e.g. polar regions for cold condition testing. These tests are often necessary to ensure that the vehicle will have a reliability and behavior that fulfills expectations of the manufacturer and also of the user. However, the areas providing suitable test conditions are often rural and remote, with high costs associated with carrying out desired tests. It is therefore desired that at least part of the tests can be performed without the need for relocation to such rural regions. This is made possible by test cells where the temperature e.g. can be controlled to desired temperatures. For example, temperatures in the order of 30-40 degrees below zero can be obtained in such test cells, thereby enabling various vehicle, as well as fuel, behaviors to be tested at low temperatures without the need for actual relocation to colder environments.
These extreme conditions in the test cell, however, provide challenges also to the equipment being used for testing the vehicle. For example, accurate testing of vehicles having advanced drive trains such as e.g. hybrid vehicles and electrical vehicles, may require dynamometer test units being capable of absorbing power from, as well as providing a propelling power to, wheel shafts of the vehicle. This can be accomplished in various ways, e.g. by the use of an electrical machine. Electrical machines are capable providing both braking and propelling torque with high accuracy, and are consequently suitable for use in testing of various vehicle functions. For example, e.g. a hybrid drive vehicle can be tested not only for accelerations, but also for complete test drives, with uphill as well as downhill driving.
However, use of electrical machines in low ambient temperatures, e.g. ambient temperatures below 0° C., −10° C. or −20° C., such as e.g. temperatures in the interval 0° C.-−50° C.; −10° C.-−50° C.; or −20° C.-−50° C., for testing e.g. vehicle behaviour in cold weather conditions, imposes high requirements on the machines in order to work properly at these temperatures. This, in turn, results in costly vehicle dynamometer systems due to requirements of especially designed machines. According to the present invention, it is provided a method for use in dynamometer testing of vehicles that allows use standard electrical machines, such as three-phase asynchronous induction motors, without requirements of being capable of operating at low temperatures according to the above.
This is accomplished by heating the electrical machine prior to commencing testing by applying a current to a stator winding of the electrical machine when some suitable temperature is below a temperature limit. This has the advantage that the electrical machine can be heated to, or be arranged to maintain, temperatures allowing desired operation while still being installed in environments in which the electrical machine is not designed to work. The invention consequently allows heating of the electrical machine from a lower temperature to operating temperatures prior to commencing testing of the vehicle so that the temperature of the electrical machine can be maintained or increased by means of said heating.
With regard to said first temperature being used for evaluating the need for heating of the electrical machine, this temperature can, for example, be an ambient temperature of the machine, hence a temperature representing surroundings of said electrical machine. This temperature can be measured at some suitable location e.g. inside a test cell in which the electrical machine is located. Alternatively, the temperature can be a temperature representing a temperature of said electrical machine, e.g. measured at some suitable location in or at the electrical machine. As is further explained below, said first temperature can also be an estimated temperature, e.g. representing an average temperature of the electrical machine, or some representing some other suitable that is not being directly measured through the use of temperature sensors. According to one embodiment, a combination of one or more temperatures can be used to determine the need for heating of the electrical machine. Said first temperature can e.g. be a temperature equal to or below one of: ten degrees Celsius, five degrees Celsius, zero degrees Celsius.
Said first temperature can also be dependent on the temperature interval in which the electrical machine is designed for operation, and e.g. be a temperature below which the electrical machine will operate outside the temperature interval in which the electrical machine is designed for operation, or a temperature being some suitable temperature above the lower limit of the temperature interval in which the electrical machine is designed for operation.
When a current is applied to a stator winding, this stator winding will be subjected to a high temperature increase due to the applied current. This temperature increase, however, may not be representative of the heating of the electrical machine when taken as a whole. That is, the stator winding temperature will quickly reach higher temperatures than other portions of the electrical machine.
This, in turn, means that if the temperature of the stator winding alone, or a temperature measured at a location being directly subjected to the increased temperature of the stator winding, is used as a representation of the machine temperature, the machine temperature might be determined as being higher than what is actually the case. In particular, there is a risk that a determination would be made, where it is concluded that the machine temperature is relatively high while in reality the average machine temperature still is low.
Consequently, there is a risk that the electrical machine would be determined as heated to a satisfactory extent while in reality this is not the case. Furthermore, there is oftentimes a maximum stator winding temperature that should not be exceeded in order to avoid temperature dependent wear/damages.
According to one embodiment of the present invention, therefore, a representation, or model, of the heat transfer in the electrical machine is used to estimate a machine temperature that more accurately reflects the actual average machine temperature. As was mentioned above, this estimated temperature can, for example, be the average temperature of the electrical machine or some other suitable temperature.
The use of a representation of the heat transfer in the machine makes it possible to estimate the manner in which supplied energy, converted to heat in the windings, changes the machine temperature with time. This also allows an accurate average machine temperature to be determined. With regard to the representation, any suitable representation can be used, such as a thermodynamic model of the electrical machine or other suitable mathematical model or a representation being determined by empirical measurements.
When using an empirical representation of the heat transfer in the machine, for example, the machine temperature in various locations can be measured and monitored as a function of supplied energy to establish the manner in which the heat transfer in the machine takes place so that an accurate representation of the temperature variations of the machine as a function of supplied energy can be obtained.
According to one embodiment, a current is applied to a stator winding until the stator winding temperature, or the temperature of some other location being directly subjected to the heat generated in the windings, reaches some temperature limit, e.g. a temperature limit that is determined from a wear/damage point of view according to the above, and when this temperature has been reached, the supply of energy is interrupted until the stator winding temperature has fallen to some other suitable temperature, e.g. until the temperature has fallen a number of degrees and/or fractions of degrees in the range 0,1-10 degrees, whereupon the current is again applied until said first temperature is reached. This can be repeated until it is determined, by means of the representation of the heat transfer of the machine, that the machine has reached the desired temperature.
The heat being generated in the stator winding will be transferred through the machine in dependence of the materials being used in the machine and the specific heat capacity of the materials. Further, the heat will be transferred towards the outer housing of the machine where it will be dissipated through cooler ambient air. Hence, the heating of the machine will be dependent on the ambient air temperature as well. The cooling off of the machine is also dependent on the design of the machine, and surface area being exposed to the surrounding air. These factors can be taken into account by the representation of the heat transfer in the machine so that inner machine temperatures can be accurately determined.
The current intensity of the current that is applied to the stator winding can be arranged to always be the same, or alternatively the current intensity can be determined based on said one or more temperatures. The applied current can also be arranged to vary during heating of the electrical machine, and e.g. be larger in the beginning of the heating to be reduced as the temperature of the machine increases.
According to one embodiment, a combination of one or more temperatures is used to determine a suitable current to be applied to said stator winding.
The applied current is preferably such that the rotor is not rotated, i.e. standing still. This can be accomplished by supplying a DC current to one or more of the phase windings of the stator winding. The electrical motor can also be e.g. a three phase motor having three phase windings and not being capable of starting when only one phase winding is being powered. In such situations, the phase winding can be provided with an alternating current and still not being able to start while still being heated.
In general, heating is obtained as a result of the fact that the efficiency is not 100%. However, when no work is produced, the supplied energy is converted to heat. According to the present invention, a zero-frequency torque can be applied, that is, a holding torque can be applied, i.e. a torque that must be overcome in order to set the rotor of the electrical machine in motion. This holding torque can be high, and when the applied torque is not taken out as mechanical work, the power providing the holding torque will be converted to heat in the windings, instead.
According to one embodiment, the heating of the electrical machine is arranged such that the rotor is not necessarily standing still but the rotational speed of the rotor is kept below a first speed. This can be accomplished by applying a current having a frequency such that the rotational speed of said rotor is being kept below a first speed, e.g. 10 rpm or 100 rpm. This can be accomplished e.g. by means of a frequency inverter. In this case, all phase windings can be powered, although by a current having a low frequency.
When heating the electrical machine it can be determined whether a temperature of said electrical machine is above a second temperature, said second temperature being equal to or higher than said first temperature, and reducing said first current when said temperature of said electrical machine is above said second temperature. The current can be reduced to any suitable current, e.g. 0 A, the heating hence being turned off.
The method according to the invention can also include continuously controlling said first current such that said temperature of said electrical machine is kept at or above said first or second temperature.
Said second temperature can, e.g., be some suitable temperature in the temperature range in which the electrical machine is designed to operate, and e.g. be a temperature above zero degrees Celsius, above five degrees Celsius, or above ten degrees Celsius. The second temperature can, for example, be determined through the use of a representation of the heat transfer in the electrical machine.
The resulting supplied power can be substantial, and e.g. be in the order of 1-100%, or 10-100% of the nominal power of the electrical machine.
The dynamometer test unit may also be of a kind having two (or more) dynamometer power sources for providing power to a same wheel shaft of the vehicle, at least one of said power sources being an electrical machine. This kind of dynamometer test units may allow a more favourable design with respect to cost/space/infrastructure requirements than would be the case with a single power source having the total capability of the two dynamometer power sources taken together.
Further features of the present invention and advantages thereof will become clear from the following detailed description of illustrative embodiments and from the attached drawings.